There is no legal basis for making any extra payments to the EU
There are some on the continent who seem to think the UK will have to pay to leave the EU, based around negotiations over how much of the continuing liabilities of the EU the UK must pay. This is all nonsense.
There is no power in the EU Treaties to impose an additional one off levy on a state as it leaves the EU. Nor is there any power in the Treaty to demand any continuing budget contributions after departure. This is wise, as of course once a state leaves it leaves behind the judicial authority of the EU which would be the means of enforcing any such payment. Article 50 is clear. Once the state leaves it has no further rights and benefits, and no further duties or obligations.
It is of course true the Treaty does not prevent the EU accepting a payment volunteered by a departing state if it wished to pay one. However, the UK could not make such a payment legally under our own law and system for controlling public spending. Ministers can only authorise spending and sign cheques for approved expenditure under UK legislation and with Parliamentary authority for the budget provision that covers the payments. Ministers have proper authority to make the annual contribution payments to the EU, required by the Treaty as incorporated into UK law by the European Communities Act. They have absolutely no authority to make one off additional payments to the EU, and would have no authority to make contributions after we have left and have repealed the 1972 Act.
They will also find that if they wanted to make a payment as overseas aid to the EU it would not qualify under our Aid budget criteria, as the EU as a whole is too rich. The only way UK Ministers could authorise a leaving payment would be to put through an Act of Parliament specifically authorising such an ex gratia payment. I can’t see many Conservative MPs wanting to vote for that.
Being in the EU is a bit like being a student in a College. All the time you belong to the College you have to pay fees. You have to obey all the rules of the institution. When you depart you have no further financial obligations, and you no longer have to obey their rules and accept their discipline. If you liked the College rules you can still apply them to yourself voluntarily. The College does not on your departure say we have borrowed money to improve the College while you were her so you will have a continuing bill for servicing the College debts. It does not say we failed to make proper provision for the future pensions of the people who taught you, so we will send you additional bills for their pensions. All your rights to reside and learn at the College cease, and all your duties to pay and obey cease. So it is with a country’s membership of the EU.